U.S. Patent Publication No. U.S.2004/0063412 discloses a method employing attenuation of a received radiofrequency signal. This document describes a radio with a RSSI (received signal strength indicator) circuit to determine if the received signal exceeds a (high) power threshold and with attenuation through second antenna signal path (through the transmit/receive switch) to avoid saturation of the receiver that would complicate the recapture of data. This RSSI based antenna selection between first (non-attenuated) and second (intentional attenuated) antenna signal enables a radio with a better reception in case of a nearby transmit source. Such antenna selection approach with RSSI above a threshold does not react on interference introduced in the radio front-end itself.
U.S. Patent Publication No. U.S.2004/001885 discloses a wireless communication circuit architecture. Antenna diversity selection is disclosed with selecting one out of two antennas for receiving while one of these two is used for transmission and it mentions antenna selection based on best quality signal. No intentional attenuation of the received signal is disclosed.
Radio systems based on a standard like IEEE 802.15.4 (/ZigBee), IEEE 802.11 (/Wi-Fi) or IEEE 802.15.1 (/Bluetooth) are commonly known. These radio systems are provided with one or more antennas of a low-power and low-cost communication device which operates in an ISM band (Industrial, Scientific and Medical).
Radio systems that operate in the 2400-2483.5 MHz ISM band can be interfered by other radio's that are nearby and can cause high interference levels on nearby frequencies. Low-cost ISM band based radio's are applied for 802.15.4/RF4CE (Radio Frequency for Consumer Electronics) remote control applications and different system radio's are used in each other vicinity. With different 2.4 GHz based radio's in the same apparatus as a TV, set-top box or laptop computer the RF isolation between the two radio's could be in the order of magnitude of 20 dB, which corresponds to an equivalent of an interference distance of 10 cm.
These radio systems have in common, a radio front-end receiver design with some band filter and LNA (low noise amplifier) which are followed by mixer and channel filter. The system allows a large range of receive levels and interference to a certain extent. With low-cost, low-power radio's the cost and power consumption, the receiver front-end is not dimensioned for linearity over a very large input level range and the allowance of very high interference levels without spectrum regrowth by non-linear distortion. Spectral regrowth is an increase in the level of a range of frequencies that develops on each side of the carrier frequency (similar to side bands) and is caused by the LNA when it has to amplify an input signal with relative high amplitude toward saturation of the LNA. This causes intermodulation distortion, which generates the so-called spectral regrowth. With non-linear distortion the third order component leads to the spectrum regrowth of the 2.4 GHz interference signal in question. Non-linearity and signal degradation by the LNA can be characterized by the 1 dB compression point (the input level at which the amplifier output shows a degradation in the output level of 1 dB) and the third order intercept (the input level at which in the output of third order component becomes equal to the output of the first order component).
FIG. 1 illustrates the power spectrum shapes of an example of a desired 802.15.4 signal with channel frequency of 2475 MHz (802.15.4 channel number 25) (by a solid line) and the related 802.15.4 filtering in the receiver (by a dashed line), and an example of an 802.11n/20 MHz interference signal with a channel frequency of 2447 MHz (802.11 channel number 8) (by a solid line) and an example of the spectrum regrowth of this interference signal inside the victim 802.15.4 receiver (by a dashed line) due to high level input at the input of the LNA. Thus, a high level 802.11n signal will result in spectral regrowth in the output signal of the LNA. The spectral regrowth extends into the adjacent 802.15.4 signal band. Consequently, the spectral regrowth interferes with communication in the 802.15.4 signal band resulting in decreased signal to noise ratio (SNR) or signal to interference ratio (SIR).
Antenna diversity, also known as space diversity, is any one of several wireless diversity schemes that use two or more antennas to improve the quality and reliability of a wireless link. The GreenPeak Emerald GP500C, 2.4 GHz ZigBee-ready IEEE 802.15.4 Communication Controller comprises a built-in antenna diversity circuit. However, antenna diversity uses the strength or amplitude or signal-to-noise ratio (SNR) of the received radio signal to determine which antenna should be selected. If the desired radio signal at the output of the LNA comprises interference components due to spectral regrowth of a neighbouring wireless link, this increases the amplitude of the received radio signal resulting in selection of the antenna signal with the highest interference and thus lowest signal quality. Similarly, if a radio signal in a neighbouring frequency band of a desired radio signal channel is transmitted, both antennae will receive the strong radio signal of the neighbouring wireless link resulting at in spectral regrowth at the output of the LNA and interference in the desired frequency channel. Consequently, irrespective which antenna will be selected, the signal-to-noise ratio is decreased due to the spectral regrowth at the LNA.